Thank you everyone who's responded. There's been a common theme in some of the responses that I wanted to clarify since I wasn't clear in my original post. This is an upgrade that will replace their current hardware as part of our standard upgrade cycle. We aren't installing this in a data center; it's local hardware meant to run just that site. It's a net negative in heat generation compared to what they currently have and we still hook into the EPO lines, so they have full control of shutdown in an emergency. Despite this clarification, I think the responses have been on point and it's given me enough to come up with some options we can work with.

I'm installing a server rack at a client site that has two APC SMARTUPS 6K UPSs in the bottom of it. I'm getting pushback from the facilities manager at that local site. This person is claiming that since the server room I'm going to install in has its own room UPS, I will cause damage to the components when I plug my UPSes into the room. When asked for details, they are claiming that plugging in a UPS to a room with a built-in UPS will cause damage due to power harmonics.

My knee-jerk reaction is of course that won't happen. However, I'd like to be able to speak intelligently about this and if I'm off base then I'd like to know too. I tried to do some research into the topic myself and went down the rabbit hole pretty quick into electrical design that's beyond my knowledge.

What may this person be talking about? Are they accurate and is it something I need to worry about?

  • Without comment on their policies (just due to inexperience in dealing with data centers), as a former industrial electrician who worked in power generation and distribution I've never heard of "power harmonics". From an electrical theory perspective, there will be little discernible difference from a load perspective between a UPS and any other device. Both have some form of a transformer connected to a rectifier, which is used to charge a battery, power downstream loads, or just power itself. The UPS won't be doing its job if it is powering upstream loads or not isolating downstream loads.
    – Paul
    Jul 15, 2021 at 14:03
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    Either change location or eat he food they serve. I would consider moving, if I get the feeling that they have no clue what they talk about (power harmonics is not a thing, but a markething). Esoteric non-sense only gets worse, in case something goes wrong. If the central UPS is not what your customer wants and it cannot be circumvented, it is basically an unwanted service forced upon your customer. Let him decide what to do.
    – moestly
    Jul 15, 2021 at 16:42
  • In theory there might be interactions, but they would be in the form of your UPS not tolerating the power provided by the room UPS, and if your UPS is not accepting it, it’s probably bad for the rest of your hardware as well. Jul 16, 2021 at 2:31
  • @Paul You've never heard of harmonics? You should have heard of harmonics. But maybe you've never heard of them as a reason to avoid having a UPS.
    – user253751
    Jul 16, 2021 at 9:29
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    Most UPSs don't have the cleanest power output in the world. I can easily see the output of an upstream UPS causing a downstream UPS to rapidly toggle between "line" and "battery", which in turn would cause a rapidly-fluctuating load on the upstream UPS. Not a "harmonic" in the normal sense of the word (more of a "resonance"), but not a good thing, either.
    – Mark
    Jul 16, 2021 at 22:26

4 Answers 4


Your typical UPS (i.e., excluding a double-conversion UPS), including all APC SmartUPS that I've worked with in the past, does nothing to the power except when running on batteries. It will draw a little extra power in regular use. When power is restored after an outage, it will draw extra power to charge the batteries. During a power outage they will just monitor the incoming power and wait until it is stable and back in range (e.g., 105V - 125V) before switching from battery to line power. If there is a room/building UPS, your own UPS will never switch to battery power. (Two exceptions: if everything is totally out (in which case, in my opinion, it doesn't matter much because you have no internet connection) or if you have a separate non-UPS power source - i.e., the data center provides both backed up and non-backed up power to your rack. That last possibility seems a bit odd to me, but based on comments it could be a thing in some places.)

So the end result is that from a practical standpoint - energy usage, interference, etc. your UPS is effectively dead-weight if there is a room/building UPS (which itself is hopefully backup by a generator for long-term outages). The only real advantage to your own UPS is that you can setup monitoring (APC has this as a standard feature) to do an orderly shutdown, but that will only apply if the room/building UPS stops working.

However, safety is an important thing. The big red shutdown button (or equivalent) for use in a true emergency - e.g., fire or flood - will shutdown power to everything else but not to your computers. Which means if your rack is part of a fire then you have a serious problem, because power will keep going. Or if everything is getting flooded, you are now pumping power into the rising waters when everything else has been safely shut off.

Plus, unlike your own office where the rack might contain your router, fiber/cable modem, etc., in a data center you are dependent on the provided network connections. If power goes out to everything else, continuing to run your computer might save some data corruption but won't actually keep your web server accessible.

So follow their directions - even though their stated reasons are meaningless.

  • Thanks for your response. I get what you're saying. We do use the monitoring functions of the UPS. Without it, this would be the only site we don't have vision into in that regard. The second thing is that we still hook up the EPO lines so if they do end up hitting the big red button, our stuff still will still shut down. Either way though, thanks for the additional information.
    – KazeEnji
    Jul 16, 2021 at 14:53
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    The problem is you won't get the monitoring because the UPS will only "know" the power is out when the building UPS/generator doesn't work. At which point, you have no internet connection, so you won't know remotely, though it would still (in theory) allow for an orderly OS/database/etc. shutdown. Jul 16, 2021 at 14:55
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    "If there is a room/building UPS, your own UPS will never switch to battery power." - this is FACTUALLY INCORRECT. I do that and for a good reason. I have a main server room USV, but a separate USV handling a small server that does backups. Not only does this isolate this server in case of a problem on the main usv output side, it also means that on a long interruption that one server keeps up another 2 hours. Data centers ALSO have issues at time.
    – TomTom
    Jul 16, 2021 at 15:07

Every data center/server room I've ever been in and worked in, that had its own UPS system, has prohibited customers, clients, etc. from using their own UPS systems. I can't speak to power harmonics, or any other reason they gave you, but they are correct. You should not install your own UPS systems.

Why would you need your own UPS system if backup power is already provided for you?

Additionally, think of yourself as a "guest". It's not your house; it's their house. Abide by their rules and respect their wishes.

See the accepted answer to this question:

What are the pros and cons of having your own UPS attached to your own server hosted in a data center?

  • 7
    As a former industrial electrician, I completely agree with the answers there. When you have to train people that in an emergency do x, except with this one single room do x and y, guess what doesn't get done, and then someone gets shocked, which is far more serious when there is already an ongoing (equipment) casualty.
    – Paul
    Jul 15, 2021 at 14:03
  • Well, you can abide by their wishes, or you can go to a different house. The point of the question is to figure out whether this is a reasonable policy, or one that is risky enough that you should consider a different host.
    – amalloy
    Jul 16, 2021 at 7:13
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    Thank you for your response. This is the only site we're responsible for that has a built-in UPS. All of our other servers are standard built and purchased as is. No real option in this case to NOT have the UPS's installed. We could either remove them completely, in which case they would have to provide a ton of L6-20 and L6-30 connections, which they don't have. Or we just leave the UPS's in bypass mode and they become fancy power strips lol.
    – KazeEnji
    Jul 16, 2021 at 14:44
  1. Fire safety.
  • The datacenter operators should be able to kill the power to the room, should an emergency happen. Your UPS units are outside of their control.
  • Batteries themselves are dangerous. A faulty battery can release its energy rather quickly (e.g. catch fire or explode) and bring an amount of damage and liability proportional to the valuables present around. A trade you may be OK with in your own data room, but the datacenter has their own priorities.
  • Acid fumes. They happen sometimes, too.
  1. Power stability issues

UPS units are pretty bad in regard to the power grid management.

  • They like to transfer their whole load between the mains and the batteries at once.
  • They have sometimes poor power factor.
  • Some of them are rather noisy in harmonics and other high frequencies that may interfere with the data center power management

These things are acceptable in a home or office, but may be rather unpleasant in a datacenter environment consisting of thousands of units of power-picky equipment.

  1. Heat management

UPS units consume ~15% of their output rating for their own needs, for no aparent gain.

On the top of everything, an average datacenter has the level of power reliability that no single UPS unit can offer.

  • Thanks for the response. I should have been more clear in this regard but I am not installing this in a datacenter. This is a rack that's upgrading and replacing the current hardware at one of our roughly 30 sites that we're responsible for as part of their standard upgrade cycle. The operators can still kill the power because we still hook up the EPO lines to the UPSs in the rack. Heat is definitely a concern but the rack we're installing is still a BTU net negative compared to what they have now even with the UPSs. I get what you're saying though and I appreciate the help.
    – KazeEnji
    Jul 16, 2021 at 14:57
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    They have no direct means of shutting down the UPSes in the rack and this is what is important in regard to the fire safety. Not really important in a small server room with 1-5 racks, but pretty much a deal breaker in more than 20 racks setup. Then again, we do have a rack with its own UPSes in a local datacenter. They were clear that either we remove our UPS or we get a non-UPS-backed (but still diesel-backed) power and that was it.
    – fraxinus
    Jul 16, 2021 at 15:14

Aside from the practical problems, most UPS manufacturers, including APC advise you not to do it and it will void your warranty. Additionally, using equipment not as designed may violate local electrical codes.

  • 1
    Pretty much legitimate argument and I wonder why the downvote. When we talk about the electricity, the Code has the proper and final say.
    – fraxinus
    Jul 16, 2021 at 17:24
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    point 4 of the APC explanation is also important: " If you are using a UPS that outputs a step-approximated sine wave when on battery, as soon as the fist UPS goes on battery, the second UPS will also go on battery because it will see the step-approximated sine wave as distorted or bad power." This may also cause your UPS to start switching between its own batteries and the room-UPS again and again. Causing problems for the output quality of the room UPS.
    – ontrack
    Jul 17, 2021 at 11:32

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